There were so many Japanese manufacturers who imported their ceramics to the US through an importing company. Most are undocumented, in regards to their maker or importer.
I would guess the only way you can find out more about these is if you see an identical one on ebay that still has its importers stciker. Because these are stamped Japan in a cospicuous place I would say they are not worth a lot of effort.
Maybe if they sink they are some type of aquarium item, the kind that sat on the bottom of the tank. That would explain the mark in an obvious location (that would not be seen in an aquarium) and the bright colors.
